A common technique for error detection and correction in packet transmission systems over unreliable channels is called Hybrid Automatic Repeat Request (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
If an FEC encoded packet is transmitted and the receiver fails to decode the packet correctly, wherein errors are usually checked by a Cyclic Redundancy Check (CRC), the receiver requests a retransmission of the packet. Generally, and throughout this document, the transmission of additional information is called “retransmission (of a packet)”, although this retransmission does not necessarily mean a transmission of the same encoded information, but could also mean the transmission of any information belonging to the packet (e.g. additional redundancy information).
Depending on the information (generally code-bits/symbols), of which the transmission is composed of, and depending on how the receiver processes the information, the following hybrid ARQ schemes are defined:    Type I: If the receiver fails to decode a packet correctly, the information of the encoded packet is discarded and a retransmission is requested. This implies that all transmissions are decoded separately. Generally, retransmissions contain identical information (code-bits/symbols) to the initial transmission.    Type II: If the receiver fails to decode a packet correctly, a retransmission is requested, where the receiver stores the information of the (erroneously received) encoded packet as soft information (soft-bits/symbols). This implies that a soft-buffer is required at the receiver. Retransmissions can be composed out of identical, partly identical or non-identical information (code-bits/symbols) according to the same packet as earlier transmissions. When receiving a retransmission, the receiver combines the stored information from the soft-buffer and the currently received information and tries to decode the packet based on the combined information. The receiver can also try to decode the transmission individually, however, generally performance increases when combining transmissions. The combining of transmissions refers to so-called soft-combining, where multiple received code-bits/symbols are likelihood combined and solely received code-bits/symbols are code combined. Common methods for soft-combining are Maximum Ratio Combining (MRC) of received modulation symbols and log-likelihood-ratio (LLR) combining, wherein LLR combining only works for code-bits.            Type II schemes are more sophisticated than Type I schemes, since the probability for correct reception of a packet increases with receive retransmissions. This increase comes at the cost of a required hybrid ARQ soft-buffer at the receiver. This scheme can be used to perform dynamic link adaptation by controlling the amount of information to be retransmitted. E.g. if the receiver detects that decoding has been “almost” successful, it can request only a small piece of information for the next retransmission, that is, a smaller number of code-bits/symbols than in previous transmission, to be transmitted. In this case it might happen that it is even theoretically not possible to decode the packet correctly by only considering this retransmission by itself (non-self-decodable retransmissions).            Type III: This is a subset of Type II with the restriction that each transmission must be self-decodable.
A new feature, the so called FDD Enhanced Uplink Dedicated Channel (E-DCH), which is also referred to as High Speed Uplink Packet Access (HSUPA), was introduced in 3GPP Release 6 (3rd Generation Partnership Project; available at http://www.3gpp.org) with the goal to improve uplink data transmission by reducing delays, increasing uplink capacity as well as uplink coverage particularly for packet data services.
Due to the rapid growth of IP-based applications there is a need for the design of a high-speed wireless data packet communication system. E-DCH aims at improving the performance of dedicated uplink (UL) transport channels. Several new techniques have been introduced in order to meet the requirements for E-DCH, as for instance is described in the Technical Specification 3GPP TSG RAN WG2 TS25.309, FDD Enhanced Uplink; Overall Description; Stage 2 (Release 6) V.6.3.0:                Node B controlled scheduling        fast HARQ protocol        shorter Transmission Time Interval (TTI)        
Node B controlled scheduling allows for a fast allocation of resources among users in the cell, which results in a better control of the uplink interference. This in turn improves the uplink coverage and cell throughput. The introduction of an HARQ protocol allows for rapid retransmissions of erroneously received data packets, thereby reducing delays caused by higher layer retransmissions. The introduction of a shorter Transmission Time Interval (TTI) allows for a further significant reduction of the overall delay and hence improves the quality of service experienced by the end user, as this is described in: Janne Peisa, Hannes Ekström, Hans Hannu, Stefan Parkvall, End-to-End Performance of WCDMA Enhanced Uplink, VTC Spring 2005, Stockholm, Sweden.
Node B controlled scheduling is one of the technical features for E-DCH which is foreseen to enable more efficient use of the uplink resource in order to provide a higher cell throughput in the uplink and to increase the coverage. The term “Node B controlled scheduling” denotes the possibility for the Node B to control, within the limits set by the RNC, the uplink resource, E-DPDCH/DPCCH power ratio, which a User Equipment (UE) may use for uplink transmissions on the E-DCH. The Node B controlled scheduling is based on uplink and downlink control signaling together with a set of rules on how the UE shall behave with respect to this signaling.
In the downlink, a resource indication, the so called Scheduling Grant, is required to indicate to the UE the maximum amount of uplink resources it may use. When issuing Scheduling Grants, the Node B may use Quality of Service (QoS)-related information provided by the Serving Radio Network Controller (SRNC) and from the UE in the Scheduling Requests.
Scheduling grants are signaled in the downlink in order to indicate the maximum resource the UE may use for uplink transmissions. The grants affect only the selection of a suitable transport format for the transmission on the E-DCH (E-TFC selection), they do not influence the TFC selection for legacy DCH channels.
There are two types of scheduling grants which are used for the Node B controlled scheduling: absolute grants and relative grants.
The absolute grants provide an absolute limitation of the maximum amount of UL resources the UE may use for uplink transmissions; they are used to rapidly change the allocated UL resources. Relative grants are transmitted every TTI. They are used to adapt the allocated UL resource, indicated by absolute grants, by smaller adjustments. The relative grant indicates the UE to increase or decrease the previously allowed maximum UL resource by one step.
Absolute grants are only signaled from the E-DCH serving cell. Relative grants can be signaled from the serving cell as well as from a non-serving cell. The E-DCH serving cell denotes the entity, which actively allocates UL resources to UEs controlled by this serving cell, whereas a non-serving cell can only limit the allocated UL resources, set by the serving cell. Each UE has only one serving cell.
Absolute grants contain the identity (E-RNTI) of the UE (or group of UEs) for which the grant is intended, the maximum power ratio the UE is allowed to use, and a flag which indicates whether the absolute grant is valid for only one HARQ process or for all HARQ processes. As already mentioned, absolute grants can be valid for a single UE, which is referred to in the following as dedicated grants, or for a group (all) of UEs, which is in the following referred to as common grant. Up to two identities (E-RNTI), a primary and a secondary, can be allocated to a UE at a time.
Relative grants can be sent, as already mentioned before, from a serving cell as well as from a non-serving cell.
A relative grant signaled from the serving cell could indicate one of the three values “UP”, “HOLD” and “DOWN”. “UP” respectively “DOWN” indicates the increase/decrease of the previously maximum used UL resource (maximum power ratio) by one step. Relative grants from a non-serving cell can either signal a “HOLD” or “DOWN” command to the UE. As mentioned before, relative grants from non serving cells can only limit the UE allowed UL resources set by the serving cell (overload indicator), but not increase.
In order to enable Node B to make an efficient scheduling, which considers also the QoS requirements of a service mapped on the E-DCH, UE provides the Node B with information by means of rate request signaling.
There are two kinds of rate request signaling in the uplink, the so called “happy bit”, which is a rate-request related flag on the E-DPCCH and the scheduling information (SI), which is sent in-band on the E-DCH.
From a system point of view, the one-bit rate request is likely to be used by the serving cell to make small adjustments in the resource allocation, for example by using relative grants. Scheduling information is instead likely to be used in making longer term scheduling decisions, which would be reflected in the transmission of an absolute grant. Details on the two rate request signaling methods are provided in the following.
As mentioned before, the scheduling information should provide Node B information on the UE status in order to allow for an efficient scheduling. The SI is included in the MAC-e PDU. The information is sent periodically to Node B in order to allow for keeping track of the UE status. The scheduling information comprises following information fields:                Logical channel ID of the highest priority data in the scheduling information        UE buffer occupancy (in Bytes):        Informs about the buffer status for the highest priority logical channel with data in buffer and the total buffer status.        Power status information:        The power status indicates the ratio of the maximum UE transmission power and the corresponding DPCCH code power.        
Identifying the logical channel by the logical channel ID from which the highest priority data originates would enable the Node B to determine the QoS requirements, e.g. power offset of the corresponding MAC-d flow, logical channel priority or GBR attribute, of this particular channel. This would in turn enable the Node to determine the grant needed to transmit the data in the UE buffer, making a more precise grant allocation possible. In addition to the highest priority data buffer status, it is beneficial for the Node B to have some information on the total buffer status. This would help in making more long-term resource allocation decisions.
In order for the serving Node B to be able to allocate uplink resources effectively, it needs to know up to what power each UE is able to support. This information could be conveyed in the form of a “power headroom” measurement, indicating how much power the UE has left over on top of that used for the DPCCH. The power status report could also be used for the triggering of a TTI reconfiguration, switching between 2 ms and 10 ms TTI and vice versa. As already mentioned before Scheduling information is sent from the UE in order to allow Node B to send a Serving Grant. In the case where the UE has no Serving Grant available and it has scheduled data to send on a logical channel the UE shall send Scheduling Information to the Node B in order to request for a Serving Grant. In case UE has a already a Serving Grant, it should send Scheduling Information to the Node B periodically in order to keep Node B up-to-date of its data/power status. This enables Node B to update the Serving Grant accordingly.
As already explained above the happy bit denotes a one-bit rate request related flag, which is sent on the E-DPCCH. The “happy bit” indicates whether it is happy or unhappy with the current Serving Grant (SG).
The UE shall indicate that it is “unhappy”, if all the following criteria are met:                UE is transmitting as much scheduled data as allowed by the current Serving Grant; and        UE has Power available to send at higher data rates (E-TFCs) and        Total buffer status would require more than X TTIs with the current Grants (where X is configurable).        
Otherwise, the UE shall indicate that it is “happy”.
There are two types of data transmissions for Enhanced Uplink, scheduled and non-scheduled transmissions. For scheduled data transmissions UE needs a valid scheduling grant before transmitting data on E-DCH. The usual procedure implies that UE sends a rate request to the serving Node B, by means of either scheduling information or happy bit, and upon reception of the rate request, serving Node B will allocate uplink resources by means of scheduling grants, i.e. absolute and relative grants, to the UE. In case of non-scheduled data transmission, the UE is allowed to send E-DCH data at any time, up to a configured number of bits, without receiving any scheduling command from the Node B. Thus, signaling overhead and scheduling delay are minimized. The resource for non-scheduled transmission is given by the SRNC in terms of a maximum number of bits that can be included in a MAC-e PDU, and is called non-scheduled grant. A non-scheduled grant is defined per MAC-d flow. Logical channels mapped on a non-scheduled MAC-d flow can only transmit up to the non-scheduled grant configured for that MAC-d flow. The Node B is informed about non-scheduled grants via Node B Application Part (NBAP) signaling, the UE via Radio Resource Control (RRC) signaling. There is a set of rules defining the handling of non-scheduled and scheduled data flows, which will be explained in detail in the following.
The UMTS Terrestrial Radio Access Network (UTRAN) can restrict a non-scheduled MAC-d flow to use only a limited number of HARQ processes in case of 2 ms TTI. For non-scheduled grants Node B has always to reserve the configured resources, i.e. maximum number of bits. In order to limit the amount of resources Node B has to reserve permanently, which might be fairly significant especially for the 2 ms TTI case, UTRAN (SRNC) can disable certain HARQ processes for non-scheduled MAC-d flows. The allocation of HARQ processes for non-scheduled MAC-d flows is configured via RRC signaling. The corresponding signaling is shown in the following Table 1 [from Technical Specification 3GPP TS 25.331 V6.8.0 (2005-12)].
TABLE 1MAC-d flow information in TS25.331 for HSUPAType andInformation Element/Group nameNeedreferenceSemantics descriptionE-DCH MAC-d flow identityMandatoryE-DCHPresentMAC-d flowidentity10.3.5.7eE-DCH MAC-d flow power offsetOptionalIntegerOnly allowed to be absent when already(0 . . . 6)defined for this E-DCH MAC-d flow; unitis dB.E-DCH MAC-d flow maximumOptionalIntegerOnly allowed to be absent when alreadynumber of retransmissions(0 . . . 15)defined for this E-DCH MAC-d flowE-DCH MAC-d flow multiplexing listOptionalBitstringIndicates, if this is the first MAC-d flow for(maxE-which PDU's are placed in the MAC-eDCHMACdFlow)PDU, the other MAC-d flows from whichMAC-d PDU's are allowed to be includedin the same MAC-e PDU.Bit 0 is for MAC-d flow 0, Bit 1 for MAC-dflow 1, . . .Value ‘1’ for a bit means multiplexing isallowed.CHOICE transmission grant typeOptionalOnly allowed to be absent when alreadydefined for this E-DCH MAC-d flow>Non-scheduled transmissiongrant info>>Max MAC-e PDU contentsMandatoryIntegersizePresent(1 . . . 19982)>>2 ms non-scheduledMandatoryBitstring (8)MAC-d PDU's for this MAC-d flow aretransmission grant HARQDefaultonly allowed to be transmitted in thoseprocess allocationprocesses for which the bit is set to “1”.Bit 0 corresponds to HARQ process 0, bit1 corresponds to HARQ process 1, . . .Default value is: transmission in all HARQprocesses is allowed.>Scheduled transmission grantNULLinfo
Further, UTRAN can reserve some HARQ processes for non-scheduled transmission (i.e. scheduled data cannot be sent using these processes, they are considered disabled) in case of 2 ms TTI.
Moreover, multiple non-scheduled MAC-d flows may be configured in parallel by the SRNC. The UE is then allowed to transmit non-scheduled transmissions up to the sum of the non-scheduled grant if multiplexed in the same TTI.
Scheduled grants will be considered on top of non-scheduled transmissions.
Finally, logical channels mapped on a non-scheduled MAC-d flow cannot transmit data using a Scheduling Grant
As can be seen from the rules defined above, the resource allocation from UTRAN side is separated between scheduled and non-scheduled grants. Also within the UE the allocation of resources to logical channels is done in accordance to scheduled and non-scheduled grants. Logical channels will be served in the order of their priorities until the non-scheduled grants and scheduled grants are exhausted, or the maximum transmit power is reached.
In parallel to the Enhanced Dedicated Physical Data Channel (E-DPDCH) used to carry the E-DCH transport channel (data bits), the Enhanced Dedicated Physical Control Channel (E-DPCCH) is always transmitted simultaneously. E-DPCCH is a physical channel used to transmit control information associated with the E-DCH.
The information fields on the E-DPCCH are:
                Retransmission Sequence Number (RSN) [2 bit]        “Happy Bit” [1 bit]        Enhanced Transport Format Combination Identifier (E-TFCI) [7 bit]        
The RSN field provides some HARQ related information like redundancy version and New Data Indicator. The Happy Bit is a one-bit rate request flag, as already mentioned above. The E-TFCI field indicates the data rate (coding format) used for the current transmission on E-DPDCH.
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
However, knowing that user and operator requirements and expectations will continue to evolve, the 3GPP has begun considering the next major step or evolution of the 3G standard to ensure the long-term competitiveness of 3G. The 3GPP recently launched a Study Item “Evolved UTRA and UTRAN”. The study will investigate means of achieving major leaps in performance in order to improve service provisioning and reduce user and operator costs. It is generally assumed that there will be a convergence toward the use of Internet Protocols (IP), and all future services will be carried on top of IP. Therefore, the focus of the evolution is on enhancements to the packet-switched (PS) domain.
The main objectives of the evolution are to further improve service provisioning and reduce user and operator costs as already mentioned. More specifically, some key performance and capability targets for the long-term evolution are:                Significantly higher data rates compared to HSDPA and HSUPA: envisioned target peak data rates of more than 100 Mbps over the downlink and 50 Mbps over the uplink        Improved coverage: high data rates with wide-area coverage        Significantly reduced latency in the user plane in the interest of improving the performance of higher layer protocols (for example, TCP) as well as reducing the delay associated with control plane procedures (for instance, session setup); and        Greater system capacity: threefold capacity compared to current standards.        
Further, another key requirement of the long-term evolution is to allow for a smooth migration to these technologies.
The uplink (UL) access scheme according to the long-term evolution will be described in the following. For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g. a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
The frequency resource can either be in a localized or distributed spectrum as illustrated in FIG. 3 and FIG. 4.
As can be seen from FIG. 3, localized single-carrier is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths of a localized single-carrier signal.
On the other hand, as can be seen from FIG. 4, distributed single-carrier is characterized by the transmitted signal having a non-continuous (“comb-shaped”) spectrum that is distributed over system bandwidth. Note that, although the distributed single-carrier signal is distributed over the system bandwidth, the total amount of occupied spectrum is, in essence, the same as that of localized single-carrier. Furthermore, for higher/lower symbol rate, the number of “comb-fingers” is increased/reduced, while the “bandwidth” of each “comb finger” remains the same.
At first glance, the spectrum in FIG. 4 may give the impression of a multi-carrier signal where each comb-finger corresponds to a “sub-carer”. However, from the time-domain signal-generation of a distributed single-carrier signal, it should be clear that what is being generated is a true single-carrier signal with a corresponding low peak-to-average power ratio. The key difference between a distributed single-carrier signal vs. a multi-carrier signal, such as e.g. OFDM, is that, in the former case, each “sub-carrier” or “comb finger” does not carry a single modulation symbol. Instead each “comb-finger” carries information about all modulation symbol. This creates a dependency between the different comb-fingers that leads to the low-PAPR characteristics. It is the same dependency between the “comb fingers” that leads to a need for equalization unless the channel is frequency-non-selective over the entire transmission bandwidth. In contrast thereto, for OFDM equalization is not needed as long as the channel is frequency-non-selective over the sub-carrier bandwidth.
Distributed transmission can provide a larger frequency diversity gain than localized transmission, while localized transmission more easily allows for channel-dependent scheduling. Note that in many cases the scheduling decision may decide to give the whole bandwidth to a single UE to achieve high data rates.
The uplink scheme should allow for both scheduled (Node B controlled) access and contention-based access. In case of scheduled access the UE is dynamically allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission.
However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, UEs can transmit without first being scheduled.
For the scheduled access, the Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically, the scheduler determines which UE(s) is (are) allowed to transmit, which physical channel resources (frequency) may be used, for how long the resources may be used (number of sub-frames) and which transport format (Modulation Coding Scheme (MCS)+transport block size) is to be used by the mobile terminal for transmission.
The allocation information is signaled to the UE via a scheduling grant, sent on the downlink control channel. For simplicity reasons, this channel is called LTE_HS_SCCH in the following. A scheduling grant message contains at least information on which part of the frequency band the UE is allowed to use, whether localized or distributed spectrum should be used, the validity period of the grant, and the maximum data rate. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme.
Uplink data transmissions are only allowed to use the time-frequency resources assigned to the UE through the scheduling grant. If the UE does not have a valid grant, it is not allowed to transmit any uplink data. Unlike in HSUPA, where each UE is always allocated a dedicated channel, there is only one uplink data channel shared by multiple users (UL SCH) for data transmissions. Furthermore, there is only one mode of operation for the uplink data access in LTE, the above described scheduled access, i.e. unlike in HSUPA where both scheduled and autonomous transmissions are possible.
To request resources, the UE transmits a resource request message to the Node B. This resource request message could for example contain information on the amount of data to transmit, the power status of the UE and some Quality of Services (QoS) related information. This information, which will be referred to as scheduling information, allows Node B to make an appropriate resource allocation.
Resource requests are transmitted using the contention-based access compared to the above described scheduled access. However, if the UE already has a valid grant, e.g., if a data transmission is ongoing, the resource requests updates can be transmitted using the granted resources, e.g., as part of MAC headers or MAC control PDU. Contention-based access can be seen as a special case of the normal scheduled access, where Node B assigns a physical resource to one user. In case of contention-based access a physical resource (sub-carriers) is assigned/shared to multiple UEs for uplink transmission. The allocation for the contention-based channel, also referred to as random access channel, is for example signaled on a broadcast channel, so that all UEs in a cell have access to this area.
FIG. 5 illustrates an exemplary allocation for contention-based access. The bandwidth of the random access channel depends on the estimated number of simultaneous accessing users and on the size of the messages transmitted on the channel. In the figure the Random access channel is allocated in a TDM fashion, one out of X sub-frames is reserved for contention-based access over the entire frequency band. However, it is also possible to allocate only part of the total Bandwidth band for random access in a distributed spectrum, in order to benefit further from frequency diversity.
Since the access is not scheduled, there is a probability, that multiple UEs access the random access channel simultaneously, leading to collisions. UE-specific scrambling and processing gain can be used in order to separate the various transmissions. The contention-based access should be only used for requesting resources in case UE has no valid grant assigned or for the initial access (going from idle to connected mode).
Channel-dependent scheduling should be also supported by the uplink-scheduling scheme in LTE. However, since there is no uplink transmission from non-scheduled UEs, same is not straightforward.
Node B needs to know the users uplink channel status before allocating resources by means of a channel-dependent scheduling algorithm. Therefore it was considered that UE transmits pilot bits, which are known at the receiver side, prior to the data transmission to support channel-dependent scheduling. Node B can consider the measured C/I of the pilots bits for the resource allocation.
The Node B controlled scheduled access is based on uplink and downlink control signaling together with a specified UE behavior with respect to the control signaling.
In the downlink, a resource allocation message is transmitted from Node B to the UE indicating the physical resources (time/frequency resource) assigned to this user. As already mentioned above, this allocation message, also referred to as scheduling grant, contains information on the identification of the user the resource allocation is addressed to, the reserved physical resource (time/frequency resource), some information on the maximum data rate, modulation and coding scheme and also probably some HARQ related information (redundancy version).
In the uplink, UE sends a scheduling request to the Node B when data for uplink transmission is available in the buffer. The scheduling request message contains information on the UE status, e.g. buffer status, QoS related information, power headroom information. This in turn allows Node B to make an appropriate allocation of resources considering also QoS requirements of the data to be transmitted.
In parallel to the actual uplink data transmission, UE signals data related control signaling, providing information on the current data transmission similar to the E-DPCCH signaling in Rel6 (HSUPA). This control signaling contains information on the chosen transport format (TFCI), which is used for decoding the data transmission at Node B, and some HARQ related information, e.g. Redundancy version, HARQ process ID and NDI. The exact information depends obviously on the adopted HARQ protocol. For example in a synchronous HARQ protocol there is no need to signal the HARQ process ID explicitly.
It should be noted, that the exact scheduling procedure for uplink, e.g. order of signaling message and the detailed format of corresponding scheduling related control messages, has not been decided upon yet.
To ensure orthogonality in the uplink, all UE transmissions must be time aligned at the Node B within the cyclic prefix. This is implemented by the Node B measuring on the received signal and, based on the timing accuracy, transmitting a timing adjustment command to the UE. The timing adjustment command is sent as control information using the downlink SCCH. Note that a UE not actively transmitting may be out-of-sync, which needs to be accounted for in the initial random access. This timing control information commands UE to advance or retract the respective transmit timing. Two alternatives for the timing control commands are currently considered:                Binary timing-control commands implying forward/backward of the transmit timing a certain step size x μs [x to be determined] transmitted with a certain period y μs [y to be determined].        Multi-step timing-control commands being transmitted on the downlink on a per-need basis.        
As long as a UE carries out uplink data transmission, the received signal can be used by Node B to estimate the uplink receive timing and thus as a source for the timing-control commands. When there is no data available for uplink, the UE may carry out regular uplink transmissions (uplink synchronization signals) with a certain period, to continue to enable uplink receive-timing estimation and thus retain uplink time alignment. In this way, the UE can immediately restart uplink-orthogonal data transmission without the need for a timing re-alignment phase.
If the UE does not have uplink data to transmit for a longer period, no uplink transmission should be carried out. In that case, uplink time alignment may be lost and restart of data transmission must then be preceded by an explicit timing-re-alignment phase to restore the uplink time alignment.
However, the above described scheduling and controlling schemes implicate several problems and drawbacks that will be outlined in the following. As described above, the E-DPCCH carrying control signaling related to the E-DCH is always transmitted simultaneously to the E-DPDCH. It contains information on the transport format of the current transmission (E-TFCI), which is represented by 7 bits. Since the decoding of the data channel (E-DPDCH) requires the information transmitted on the related control channel (E-DPCCH), the control information needs to be received correctly. Therefore in order to guarantee a reliable transmission, the control channel needs to be transmitted with sufficient power. Since the UE is in most cases power-limited in the uplink there is a general pursuit of reducing the amount of control signaling.
In case of HSUPA, the scheduling grant indicates the maximum power ratio, the UE is allowed to use for transmission of scheduled data. The actual used transport format, e.g. data rate, could however differ significantly from the scheduled maximum value, since E-DCH has also to compete with non-scheduled DCH traffic, which is always highest priority. Furthermore there is also non-scheduled data in HSUPA as already described above. Therefore the transport format for E-DCH transmission selected during E-TFC selection depends on the scheduling grant, other UL traffic (DCH, HS-DPCCH) and finally the power status of the UE.
This in turn makes it unavoidable, that the UE has to signal the absolute value of the used transport format (E-TFCI) of the current data transmission represented by 7 bits.
As already indicated above, also for the uplink scheme in LTE UE is required to signal the used transport format of the uplink data transmission in order to allow for correct decoding of the data packet at Node B.